An air conveyor for a sampling tube is disclosed in German Patent No. DE 333057 C2. In this device, a spring-loaded counting ring is disposed around the axis of the body and, by rotation, unlocks the control rod for releasing the suction stroke of the bellows. The control rod is locked in the body in both end positions by means of a radially extending spring-loaded stop cam disposed in the control rod. Depending upon the end position taken up, the spring-loaded stop cam engages in radial locking recesses in the wall of the body or, in the case of the end position when the bellows is vented, it cooperates with a spring-loaded locking element disposed under the counting ring. This device, however, may result in disadvantageous inaccuracies in determining the beginning and the end of the suction stroke due to the inaccuracies in stopping the control rod.
It would be desirable, therefore, to develop an air conveyor wherein the beginning and the end of the suction stroke is uniquely determined by an improved locking and releasing mechanism thereby ensuring an exactly reproducible stroke volume.